Fiber optic probes make it possible to collect spectral information such as Raman spectra without having to place the material being characterized inside a spectrometer housing. Such probes therefore simplify the interfacing of spectroscopic systems to chemical processes, and allow analytical instruments to be located remotely from hostile environments in need of spectroscopic monitoring.
Among the first remote fiber-optic probes for Raman spectroscopy were reported by the McCreery group in the early 1980's. Their design used a single optical fiber to deliver laser light to the sample and a single optical fiber to collect light scattered by the sample. More specifically, divergent laser light from the laser delivery fiber was used to illuminate the sample, and light scattered from the sample within the acceptance cone of the collection fiber was transmitted back to the spectrograph. The efficiency of exciting and collecting Raman photons from any individual point in the sample was poor, but the integrated Raman intensity over the unusually large analysis volume compared favorably with the more traditional imaged illumination and collection.
McCreery's dual fiber Raman probe offered important benefits for remote and routine spectroscopy: 1) the sample could be distant from the Raman instrument, 2) no sample alignment was necessary once the probe was aligned to the spectrograph, 3) the probe could be less than 1 mm in diameter, making Raman measurements possible for samples with limited accessibility, 4) the probe could be placed directly in hostile samples (corrosive, hot, etc.) since only silica and the encapsulation material were exposed, and 5) multiple measurements could be made simultaneously by placing multiple collection fibers along the slit height of the spectrograph.
Several improvements to the McCreery Raman probe have more recently been reported. Instead of using just one collection fiber, multiple fibers have been used to increase the collection efficiency. For example, 6 fibers, each having the same diameter as the excitation fiber, may be grouped around the excitation fiber to form a single circular collection layer, as shown in U.S. Pat. No. 4,573,761. Eighteen fibers, each having the same diameter as the excitation fiber, may also be grouped around the excitation fiber as two circular layers, and so on, though successive layers tend to be less effective at collecting Raman photons than the first layer.
The performance of the McCreery type probe can also be modified for improved collection efficiency and/or working distance by changing the overlap between the emission cone of the excitation fiber and the collection cones of the collection fibers. The first realization of this idea, reported by McLachlan, U.S. Pat. No. 4,573,761, angled the collection fibers such that their optic axes intersected the optic axis of the illumination fiber. This increased the overlap of the excitation and collection cones close to the tip of the fiber probe, where the excitation and collection of Raman photons was most efficient. The same concept was later implemented in a different way by O'Rouke, who ground the tip of the probe into a cone shape, as discussed in U.S. Pat. No. 5,402,508. This shape was equivalent to putting prisms (or more correctly, axicon sections) on the collection fibers so that their optic axes crossed the optic axis of the excitation fiber.
A serious problem with all of the fiber optic Raman probes discussed so far is probe background. Laser light inside an optical fiber generates Raman and fluorescence emission from the fiber core itself. This emission can overwhelm the desired Raman signal from the sample and is the reason that the intense excitation light must be carried by a fiber separate from the collection fiber(s). Nevertheless, emission generated in the excitation fiber can reflect from the sample into the collection cones of the collection fibers. Likewise, laser light reflected from the sample into the collection cones of the collection fibers generates the emission while propagating through the collection fibers back to the spectrograph. Symmetry arguments predict that the optical fiber emission observed at the spectrograph will have equal contributions from the excitation fiber and the collection fibers if the length of the excitation fiber is the same as the length of the collection fibers.
When a McCreery-style probe is inserted into a transparent solution, or into a homogeneous partially absorbing solution, reflection of laser light or fiber emission back to the collection fibers is negligible and the optical fiber emission is not observed. Raman spectra of solid samples tend to suffer from probe background. The impact of this background can sometimes be reduced to acceptable levels by angling the probe with respect to the sample surface normal, by eliminating the spectral region containing the fiber emission from the analysis, or by spectral subtraction of the fiber emission.
For many Raman applications elimination of the optical fiber background is at least desirable, if not necessary. The O'Rouke group and other groups have inserted a thin-film dielectric interference filter into the excitation fiber to filter out some of the optical fiber background. One way to do this was to place the filter inside an SMA-to-SMA connector and put the fibers in contact with the filter. Another way to do this was to collimate the laser output from a fiber, send the collimated beam through the filter, and focus the transmitted light back onto the other fiber. Both approaches require precise alignment and attenuate the laser energy. More importantly, the laser energy must still travel through several centimeters to several meters of fiber between the filter and the probe tip, depending on geometry constraints of the specific application. Emission from this intermediate length of fiber optic cable is still a serious problem.
In-line filters can also be added to the collection fibers, but the same limitations apply. In addition, the use of separate filters on each collection fiber rapidly becomes impractical. The use of a single filter for the collection fiber bundle requires precise rotational alignment, in addition to precise translational alignment. One other approach, coupling the collection fiber bundle to a single large fiber prior to filtering, seriously reduces optical throughput because the larger fiber couples more poorly to spectrographs.
A more effective approach for eliminating optical fiber background was described by Carrabba et al. They described an optical probe head located between the sample and the optical fibers. The probe head removed the fiber background from the excitation fiber and removed scattered laser light before the Raman signal reached the collection fiber. An improved probe head based on this concept was later described by Owen et al. An additional feature of these probe heads was the use of imaged excitation and collection. Imaged excitation and collection, compared with the non-imaged approach of the McCreery-style probes, offer smaller measurement areas on the sample, smaller depth-of-focus, and a variable working distance between the probe and the sample.
A serious problem with all of the fiber optic Raman probes discussed so far is probe background. The laser light used for sample excitation can also generate undesirable luminescence, including Raman scattering and fluorescence. It has been determined, for example, that certain materials placed in the optical collection path may exhibit undesirable luminescence when excited by the commonly used laser source, or by sample emissions, or both. It is known, for instance, that certain adhesives used in forming multilayer optical elements generate undesirable luminescence, particularly fluorescence, and that the dielectric coatings or dichromated gelatin (DCG) used to form holographic optical elements may produce unwanted signatures as well. Such emissions, if allowed to propagate through the collection fibers back to the spectrograph, can overwhelm the desired Raman signal from the sample. Thus, any technique to reduce these sources of noise would be welcomed by those seeking greater performance or accuracy from instruments incorporating these optical components.
Most fiber optic probes used for Raman 180.degree. backscattering described in the literature use an amplitude-splitting beam combiner to overlap the excitation and collection optical paths. The beam combiner is a major source of probe background. Wavefront-splitting beam combiners are commonly used in directly-coupled Raman spectroscopy instruments. Wavefront-splitting beam combiners can be designed to have negligible luminescence background. No designs for fiber optic probes using wavefront-splitting beam combiners have been reported in the literature.